Image sensors are electronic devices configured to transform optical images into electrical signals. Types of image sensors can include complementary metal-oxide-silicon (CMOS) and charge-coupled devices (CCD), among others. A unit pixel of an image sensor is generally composed of photodiodes configured to accept light, and transistors configured to control image signals received from the photodiodes. In the photodiodes, electron-hole pairs are generated with wavelengths and intensities of red, green, and blue light. Red, green and blue light signals are generated using corresponding color filters. The output signal of a unit pixel varies as a function of the quantity of electrons induced therein. In this manner, an optical image maybe sensed.
An image sensor can be divided into two general regions. The first is a pixel region, which includes the photodiodes arranged in an array, for example. The second region is a peripheral circuit region, which can include transistors configured to drive image signals originating in the pixel region. The transistors located in the peripheral circuit region can be operable at various voltage ranges. For example, transistors located in an analog area of the peripheral circuit region are generally operable at higher voltages. These transistors generally are constructed to include thick gate insulation layers for the high threshold voltages. In contrast, transistors located in a digital area in the peripheral circuit region are generally operable at lower voltages, and have thin gate insulation layers for fast response and low threshold voltages. As such, the transistors in the peripheral circuit region are generally constructed to include a variety of gate insulation layers to accommodate the different operating voltages.
In the transistors, it is common for the thick gate insulation layer to be formed of a pure silicon oxide layer and the thin gate insulation layer to be formed of a silicon oxynitride layer, which can contribute to device reliability and protection against boron diffusion. In the process of forming the gate insulation layers with different thicknesses, nitrogen particles may diffuse through the thick gate insulation layers during the formation of the SiON layers. In the case where nitrides are generated at interfaces between the thick gate insulation layers and a silicon substrate, interface traps can form due to the dangling bonds of silicon that result from the nitrogen particles. These interface traps in the image sensor incur problems such as flickering noise and can result in image quality deterioration.